1. Field of the Invention
The present invention relates to an apparatus and method for bonding at least two continuously moving webs together. The invention more particularly concerns an apparatus and method for ultrasonically bonding at least two continuously moving webs together using a rotary ultrasonic horn.
2. Description of the Related Art
Several different conventional methods have existed for bonding at least two continuously moving webs together. For example, it has been well known to those skilled in the art to bond two continuously moving substrate webs together by constrictively passing them between a rotating bonding roll and a rotating anvil roll. Typically, the anvil roll has been configured to bond the webs together in a predetermined bond pattern. The substrate webs have been bonded together by any means known to those skilled in the art such as thermal, ultrasonic or adhesive bonding. For example, the bonding roll has been heated to thermally bond the webs together as the webs constrictively travelled between the bonding roll and the anvil roll. Alternatively, the bonding roll has included a rotary ultrasonic horn which has been capable of transmitting ultrasonic energy to ultrasonically bond the two webs together as they constrictively travelled between the rotary ultrasonic horn and the anvil roll. Representative examples of rotary ultrasonic horns which have been used to bond at least two webs together are described in commonly assigned U.S. Pat. Nos. 5,096,532 to Neuwirth et al. and 5,110,403 to Ehlert.
The consistency and quality of the bonds between the webs when using rotary bonding techniques is dependent upon the force exerted on the webs by the anvil roll and bonding roll; the time that the webs are pressed together which is dependent upon the operating speed; and the types of materials being bonded together. In thermal bonding methods, the consistency and quality of the bonds has also been dependent on the temperature of the bonding roll. In ultrasonic bonding methods, the consistency and quality of the bonds has also been dependent on the frequency and amplitude of the vibrations of the ultrasonic horn.
Many of the conventional methods for rotary bonding have included a rotating anvil which is rigidly mounted. However, such conventional methods have not been sufficiently satisfactory. Although the use of a rigidly mounted, rotating anvil was a significant improvement in continuous bonding methods, such use has some inherent limitations which adversely affect the bond quality which, in turn, limits the operating speeds. When the rotating anvil is rigidly mounted, the consistency and quality of the bonds between the two webs is dependent upon the runout in both the bonding roll and the anvil roll and the amount that both rolls flex when under a variable load due to the types of materials being bonded and the variable operating speeds. In such a configuration, it has been virtually impossible to maintain proper alignment between the bonding roll and the anvil roll to achieve the desired constant force between the rolls in the bond region especially as the process variables change. Thus, in many of the conventional methods for rotary bonding, the bond quality has been undesirably variable both along the length and across the width of the bond region.
The consistency and quality of the bonds when rotary bonding using conventional methods has been particularly variable as the width of the desired bond pattern exceeds about 1 centimeter because it becomes increasingly difficult to maintain the constant force and contact between the bonding and anvil rolls across the entire width of the bond pattern. When using many of the conventional methods for rotary bonding in such a configuration, the actual percentage of the area of the webs being bonded together has been much less than the desired bond area based on the area of bond pattern on the anvil roll.
Many of the conventional methods for rotary bonding have used different approaches to diminish the extent of these limitations. For example, the bonding roll, anvil roll and support frames have been precisely machined to minimize the runout in the bonding system. In addition, the strength of the bonding and anvil rolls and their support frames has been increased to minimize the flexing under the variable load conditions. However, these approaches have been expensive and inefficient and have required extensive setup modifications as the process variables, such as operating speed, are changed.
The above-mentioned difficulties of maintaining the desired bond quality and consistency have been even more acute when ultrasonically bonding at least two continuously moving webs together using a rotary ultrasonic horn. The rotary ultrasonic horn has inherent movement which may adversely affect bond consistency and quality because it continuously vibrates at a given frequency and amplitude to efficiently bond the two webs together. Moreover, the rotary ultrasonic horn has usually been mounted in a cantilevered configuration which enhances the amount of flex under load.